1. Field of the Invention
The present invention relates to a surface emitting type optical semiconductor device. In particular, the present invention relates to a surface emitting type optical semiconductor device of vertical resonator type having a current confinement structure separated by an impurity region.
2. Related Art
In recent years, optical data transmission using an optical semiconductor device utilizing compound semiconductors for Groups III-V as a light source has been widely put to practical use. As its transmission line, a fiber utilizing silica, a fiber utilizing polymers, or spatial transmission is used. As its light source, a surface emitting type optical semiconductor device such as a surface emitting type laser or a light emitting diode of optical cavity type which is advantageous in optical coupling to fibers and cost reduction is used.
However, the conventional surface emitting type optical semiconductor device has a problem of low optical output. This problem has become especially remarkable in recent years because it is necessary to raise the optical output as the transmission rate is increased. Especially, in the surface emitting type laser, it is necessary to reduce a diameter of the current confinement portion in order to obtain high optical stability in the lateral mode. This becomes a restriction factor in increasing the optical output.
Furthermore, for increasing the optical output in a surface emitting type laser having a small device size, it is necessary to raise the injection current density. If it is attempted to increase the optical output, heat is generated in a narrow region and the temperature is apt to rise. This results in a problem that the device characteristics are degraded by the heat generation. For solving these problems, it is indispensable to lower the device resistance.
In the conventional surface emitting type optical semiconductor device of ion implantation type, a high resistance region is formed by injecting protons from the crystal surface into the outside of a region serving as a current confinement region. As a result, the current confinement region in which a current is confined is formed inside the high resistance region. At this time, it is necessary to form a contact electrode on a surface corresponding to the current confinement region inside the region raised in resistance by injecting protons. In an example of a surface emitting type laser having such a configuration, a substrate side semiconductor multilayer film reflecting mirror layer is provided on a GaAs substrate. An active layer including quantum wells and spacers is provided on the substrate side semiconductor multilayer film reflecting mirror layer. In addition, a p-side electrode opened so as to provide an aperture of laser emission region with a predetermined value is provided on a surface side semiconductor multilayer film reflecting mirror layer. Furthermore, a high resistance region formed by proton injection is provided in a peripheral part of the surface side semiconductor multilayer film reflecting mirror layer in order to define a current injection region (current confinement region) to the active layer.
As for the p-side electrode in this surface emitting type laser, an ohmic junction is formed on the surface inside the surface of the high resistance region formed by proton injection as described above. This results in a problem that the optical output is lowered by the p-side electrode metal and a problem that the injected current quantity must be increased by widening the current confinement region more than needed. Furthermore, since the contact area cannot be made large, it is also difficult to make the contact resistance small. Especially in this example, it is necessary to set the aperture of the laser emission port to a small value in order to control the lateral mode. Therefore, it is extremely difficult to make the contact area large.
For example, Japanese Patent Application Laid-Open Publication No. 2000-22271 exemplifies a device formed by forming a high resistance region on the side parts of a semiconductor multilayer film reflecting mirror (hereafter referred to as DBR (distributed Bragg reflector) as well) in a surface emitting type laser using ion implantation and inside of the high resistance region is used as a current confinement region. This technique has improved the controllability of the basic mode in a surface emitting type laser having a single mode, and study from the viewpoint of the device resistance is not conducted. As regards the device resistance, neither a concrete relevant device structure nor its manufacturing method is described. There is only description that ions are implanted into the side parts of the DBR. Therefore it is considered that ions are implanted from the top. Therefore, it is considered that the resistance of the DBR of an upper part of the ion implantation region and a second adjustment layer formed on the DBR becomes high. This point can be guessed from that the inside diameter of an electrode provided on the second adjustment layer is smaller than the inside diameter of the ion implantation region. Because if the inside diameter of the electrode is made larger than the inside diameter of the ion implantation region, the electrode comes in contact with only a portion raised in resistance by ion implantation and the device resistance becomes great. As a result, it becomes difficult to let flow a sufficient current or heat generation increases, resulting in a lowered optical output. If the inside diameter of the electrode is made smaller than the inside diameter of the ion implantation region as described in Japanese Patent Application Laid-Open Publication No. 2000-22271, the electrode intercepts light, resulting in a problem of an insufficient laser optical output.
Furthermore, for example, in a first embodiment described in Japanese Patent Application Laid-Open Publication No. 10-56233, a high-resistance low-reflection zone using ion implantation is provided in an upper mirror, and a loss determining device is formed in an upper part of the high-resistance low-reflection zone by using epitaxial growth. If such a configuration is used, design that prevents the resistance at an electrode junction portion from increasing and optical absorption conducted by the electrode from exerting influence becomes possible. However, there is no concrete description as regards the electrical resistance of the loss determining device. It is described that it is desirable to set the resistance equal to substantially zero. Since the loss determining device is formed of GaAs, the impurity concentration is increased to the utmost limit to lower the resistance. If the device is produced on the basis of such a thought, then the carrier concentration of a compound semiconductor is high and consequently the optical absorption coefficient becomes large, resulting in a problem of an increased optical loss. Because of the position relation between the electrode and the core zone, current injection is apt to occur in a periphery portion of the core zone as compared with the inside of the core zone. In the case of laser operation, the fundamental mode is apt to become unstable. In another embodiment described in Japanese Patent Application Laid-Open Publication No. 10-56233, a current diffusion layer is provided. Since it is described that the resistance is made low as far as possible, however, it cannot be avoided that the optical loss is increased by the current diffusion layer.
When the surface emitting type laser is made to operate with a high optical output, it is necessary to inject a large current into a narrow region. Therefore, it is not easy to make the optical output of the surface emitting type laser high. Especially when the surface emitting type laser is made to operate at a high frequency, it is necessary to inject a current into the narrow region and obtain a high optical output. Since in this case the current is injected into the narrow region, the device resistance becomes high and generated heat increases. Since heat is generated in the narrow region, the device temperature is apt to rise and the optical output is lowered. Therefore, it becomes necessary to conduct further current injection. This results in a problem that a vicious cycle occurs.
Furthermore, even if a diameter of the current confinement region is made large in order to increase the optical output, it is difficult to inject a current uniformly into the current confinement region. Rather, the current density in the central part of the device falls and the optical output becomes non-uniform. This results in a problem that the optical mode becomes unstable.